Hot Jupiters Co-existing with Earth-like Worlds?

by Paul Gilster on October 29, 2007

One of the surprises of the early planet-hunting era has been the discovery of ‘hot Jupiters,’ giant planets orbiting extremely close to their parent star. That these planets should be prolific in our catalog at present makes sense given the nature (and limitations) of radial velocity detection methods, but before we started finding them, there seemed little reason to believe gas giants would exist at orbits within 0.1 AU. Now we see them as evidence that protoplanets can migrate during the formation period, probably causing havoc as they pass through the inner system.

Are hot Jupiters the bane of terrestrial planets? You would think so, given the above scenario, with a gas giant clearing planet-forming materials out of the inner disk during its passage. But Martyn Fogg and Richard Nelson (University of London) think otherwise. Their new paper looks at models of terrestrial planet formation and finds that inner disks survive the passage of the inbound giant, resuming their planetary formation once the hot Jupiter has closed to its new, searingly close orbit.

A sample scenario goes like this, using a simulation based on a system that has evolved for a million years before the giant planet appears and begins its migration:

The results show that the passage of the giant does not sweep the inner system clear of planet-forming material. Instead, the giant planet shepherds the solids disk inward, compacting it and exciting the orbits of objects captured at mean-motion resonances. Much of this excited material eventually experiences a close encounter with the giant planet and is expelled into an exterior orbit, augmenting a new disk of solid material that progressively builds up in orbits external to the ﬁnal position of the hot-Jupiter. In this particular case, 86% of the solids disk survives, with 82% of it residing in the external scattered disk.

If this work is correct, the passage of a wandering gas giant has a different aspect than we have assumed. The inner disk is only slightly diluted by the event. Moreover, the materials left behind are prompted to new planetary formation with volatile-rich material moving inward. And note this comment on orbital eccentricity (internal references deleted for brevity):

The results of further simulation of accretion in this scattered disk show that the initially eccentric orbits of protoplanets are rapidly damped and circularized via dynamical friction exerted by smaller bodies and possibly via tidal drag exerted by the remaining gas. Planetary growth resumes and over the following ~ 10–100 Myr gives rise to a set of water-rich terrestrial planets in stable orbits external to the hot-Jupiter.

Thus another paradigm shift: This paper suggests that all those hot Jupiter systems we’ve been more or less writing off for Earth-like planets are back in the game. These systems account for roughly one-quarter of all exoplanets thus far found, so the finding isn’t insignificant. Can Kepler, Darwin or perhaps a mission like ESA’s proposed PLATO track down a terrestrial world in such a system? The technology should be up to the task, but the first step is the realization, more than a little surprising, that we may need to add hot Jupiter systems to the target list.

The paper is Fogg and Nelson, “Can Terrestrial Planets Form in Hot-Jupiter Systems?” to appear in Extreme Solar Systems, ASP Conference Series, eds. Debra Fischer, Fred Rasio, Steve Thorsett and Alex Wolszczan (abstract). Personally, I’m finding the interest in extreme systems fascinating, because part of the study is figuring out which systems are actually extreme, and which scenarios, however unlike our own, may be common.

Whether the resultant planets would be terrestrial, or more like warm analogues of the outer planetary moons (“ocean planets”), I don’t know.

And bear in mind the 55 Cancri system: two inner-system Jovians (a hot Jupiter and an intermediate-period planet), a planet at Jupiter-like distances, and a large gap in-between which could harbour additional planets, indeed some theories of how planetary systems form pretty much demand that there is something in this gap. Furthermore, the habitable zone of the system is contained within the gap. Definitely one to watch.

@andy
You are right about 55 Cancri, I checked the Extrasolar Planets Encyclopedia:
a hot Jupiter and a ‘hot Neptune/Uranus’ in very close orbit, then a Saturn-sized planet at slightly less than Mercury-orbit (at 0.24 AU), then a giant Jupiter at very Jupiter like orbit (5.3 AU). So the gap between these two outer ones is huge indeed, some 5 AU !
What bothers me a bit is the fact that these two have quite eccentric orbits (0.44 and 0.33 ecc.), which may not matter much for the inner one, but how about the giant Jupiter (3.9 Mj) with 0.33 ecc?.
However, the gap may be large enough not to disturb any terrestrial planets. Surely it should be possible to computer model this (and maybe has already been done)?! Promising indeed!

For any an earth or super earth orbiting in the ~5 AU gap i.e. in between the Saturn and outer Super Jovian in the 55 Cancri system, the nightsky must be quite fascinating. In fact, the HD 168443 system could an even more breathtaking system to behold for an Earth-like world (if one exists, unlikely though) in between the inner 7.7 MJup HD 168443 b and the outer 17.2 MJup behemoth HD 168443 b. Wonder if anyone here has run any simulation to see if terrestrial worlds can occupy a stable orbit in between these 2 mega giants in the HD 168443 system hmmm…

Shaun: apparently the region between the two substellar objects in the HD 168443 system is unstable, see Barnes and Raymond (2004).

Ronald: the eccentricities do not seem to be a problem at 55 Cancri. Planet formation simulations do tend to form terrestrial-mass planets in this system, e.g. Raymond, Barnes and Kaib (2006) – provided there is surviving material to form planets left over in the gap. Furthermore, the eccentricities may not be so high: the Catalog of Nearby Exoplanets gives a low-eccentricity solution. Having had a crack at coming up with a solution for the system at the Systemic Project, I’m somewhat sceptical about this: while low-eccentricity inner planets tend to give a better solution when planet-planet interactions are taken into account, the outer planet definitely seems to be eccentric.

Well, looks like prospects for a habitable terrestrial planet around 55 Cancri have been dsahed by the presence of a sub-Saturn mass planet right in the habitable zone (bringing the total number of known planets in that system up to the record high number of 5).

Abstract: To date, two planetary systems have been discovered with close-in, terrestrial-mass planets (less than 5-10 Earth masses). Many more such discoveries are anticipated in the coming years with radial velocity and transit searches.

Here we investigate the different mechanisms that could form “hot Earths” and their observable predictions. Models include: 1) in situ accretion; 2) formation at larger orbital distance followed by inward “type 1″ migration; 3) formation from material being “shepherded” inward by a migrating gas giant planet; 4) formation from material being shepherded by moving secular resonances during dispersal of the protoplanetary disk; 5) tidal circularization of eccentric terrestrial planets with close-in perihelion distances; and 6) photo-evaporative mass loss of a close-in giant planet.

Models 1-4 have been validated in previous work. We show that tidal circularization can form hot Earths, but only for relatively massive planets (greater than 5 Earth masses) with very close-in perihelion distances (less than 0.025 AU), and even then the net inward movement in orbital distance is at most only 0.1-0.15 AU. For planets of less than about 70 Earth masses, photo-evaporation can remove the planet’s envelope and leave behind the solid core on a Gyr timescale, but only for planets inside 0.025-0.05 AU.

Using two quantities that are observable by current and upcoming missions, we show that these models each produce unique signatures, and can be observationally distinguished. These observables are the planetary system architecture (detectable with radial velocities, transits and transit-timing) and the bulk composition of transiting close-in terrestrial planets (measured by transits via the planet’s radius).

Abstract: We present results of the photometric campaign for planetary and low-luminosity object transits conducted by the OGLE survey in 2005 season (Campaign #5). About twenty most promising candidates discovered in these data were subsequently verified spectroscopically with the VLT/FLAMES spectrograph.

One of the candidates, OGLE-TR-211, reveals clear changes of radial velocity with small amplitude of 82 m/sec, varying in phase with photometric transit ephemeris. Thus, we confirm the planetary nature of the OGLE-TR-211 system. Follow-up precise photometry of OGLE-TR-211 with VLT/FORS together with radial velocity spectroscopy supplemented with high resolution, high S/N VLT/UVES spectra allowed us to derive parameters of the planet and host star.

OGLE-TR-211b is a hot Jupiter orbiting a F7-8 spectral type dwarf star with the period of 3.68 days. The mass of the planet is equal to 1.03+/-0.20 M_Jup while its radius 1.36+0.18-0.09 R_Jup. The radius is about 20% larger than the typical radius of hot Jupiters of similar mass. OGLE-TR-211b is, then, another example of inflated hot Jupiters – a small group of seven exoplanets with large radii and unusually small densities – objects being a challenge to the current models of exoplanets.

The Effect of Poloidal Magnetic Field on Type I Planetary Migration: Significance of Magnetic Resonance

Authors: Takayuki Muto, Masahiro N. Machida, Shu-ichiro Inutsuka

(Submitted on 7 Dec 2007)

Abstract: We study the effect of poloidal magnetic field on type I planetary migration by linear perturbation analysis in the shearing-sheet approximation and the analytic results are compared with numerical calculations. In contrast to the unmagnetized case, the basic equations that describe the wake due to the planet in the disk allow magnetic resonances at which density perturbation diverges. In order to simplify the problem, we consider the case without magneto-rotational instability.

We perform two sets of analyses: two-dimensional and three-dimensional. In two-dimensional analysis, we find the generalization of the torque formula previously known in unmagnetized case. In three-dimensional calculations, we focus on the disk with very strong magnetic field and derive a new analytic formula for the torque exerted on the planet. We find that when Alfven velocity is much larger than sound speed, two-dimensional torque is suppressed and three-dimensional modes dominate, in contrast to the unmagnetized case.

Abstract: This report is a review of Darwin’s classical theory of bodily tides in which are given the analytical expressions for the orbital and rotational evolution of the bodies and for the energy dissipation rates due to their tidal interaction. It is shown that almost all results found in the literature for the study of evolution due to tidal friction can be straightforwardly derived from Darwin’s theory. General formulas are given which do not depend on any assumption linking the tidal lags to the frequencies of the corresponding tidal waves (except that equal frequency harmonics are assumed to span equal lags). The general formulas are applied to several physical scenarios including both fast and slow rotating central bodies as well as their companions. Emphasis is given on the case of companions having reached one of the two possible final states: capture into a 1:1 spin-orbit resonance (synchronization) or the super-synchronous stationary rotation resulting from the vanishing of the average tidal torque. The true synchronization with non-zero eccentricity is only possible if an extra torque exists opposite to the tidal torque. The theory is developed assuming that this additional torque is produced by an equatorial permanent asymmetry in the companion. The indirect tidal effects and some non-tidal effects due to that asymmetry are considered. The theory is developed only to the second degree in eccentricity and inclination (obliquity), but can easily be extended to higher orders.

Abstract: In this paper we investigate the evolution of a pair of interacting planets – a Jupiter mass planet and a Super-Earth with the 5.5 Earth masses – orbiting a Solar type star and embedded in a gaseous protoplanetary disc. We focus on the effects of type I and II orbital migrations, caused by the planet-disc interaction, leading to the Super-Earth capture in first order mean motion resonances by the Jupiter. The stability of the resulting resonant system in which the Super-Earth is on the internal orbit relatively to the Jupiter has been studied numerically by means of full 2D hydrodynamical simulations.

Our main motivation is to determine the Super-Earth behaviour in the presence of the gas giant in the system. It has been found that the Jupiter captures the Super-Earth into the interior 3:2 or 4:3 mean motion resonances and the stability of such configurations depends on the initial planet positions and eccentricity evolution. If the initial separation of planet orbits is larger or close to that required for the exact resonance than the final outcome is the migration of the pair of planets with the rate similar to that of the gas giant at least for time of our simulations. Otherwise we observe a scattering of the Super-Earth from the disc. The evolution of planets immersed in the gaseous disc has been compared with their behaviour in the case of the classical three-body problem when the disc is absent.

Abstract: Precision radial velocity (RV) measurements of the Sun-like dwarf
14 Herculis published by Naef et. al (2004), Butler et. al (2006) and
Wittenmyer et al (2007) reveal a Jovian planet in a 1760 day orbit and a trend indicating the second distant object. On the grounds of dynamical considerations, we test a hypothesis that the trend can be explained by the presence of an additional giant planet. We derive dynamical limits to th orbital parameters of the putative outer Jovian companion in an orbit within ~13 AU.

In this case, the mutual interactions between the Jovian planets are important for the long-term stability of the system. The best self-consistent and stable Newtonian fit to an edge-on configuration of Jovian planets has the outer planet in 9 AU orbit with a moderate eccentricity ~0.2 and confined to a zone spanned by the low-order mean motion resonances 5:1 and 6:1. This solution lies in a shallow minimum of \Chi and persists over a wide range of the system inclination. Other stable configurations within 1\sigma confidence interval of the best fit are possible for the semi-major axis of the outer planet in the range of (6,13) AU and the eccentricity in the range of (0,0.3). The orbital inclination cannot yet be determined but when it decreases, both planetary masses approach ~10 Jupiter masses and for ~30 deg the hierarchy of the masses is reversed.

Comments: Revised version, 10 pages with low resolution figures suitable for arXiv, accepted to MNRAS. The manuscript with full resolution figures may be downloaded from this http URL (warning! large file, 9MB). The definitive version will be/is available at this http URL

Abstract: Both core accretion and disk instability appear to be required as formation mechanisms in order to explain the entire range of giant planets found in extrasolar planetary systems. Disk instability is based on the formation of clumps in a marginally-gravitationally unstable protoplanetary disk. These clumps can only be expected to contract and survive to become protoplanets if they are able to lose thermal energy through a combination of convection and radiative cooling. Here we present several new three dimensional, radiative hydrodynamics models of self-gravitating protoplanetary disks, where radiative transfer is handled in the flux-limited diffusion approximation. We show that while the flux-limited models lead to higher midplane temperatures than in a diffusion approximation model without the flux-limiter, the difference in temperatures does not appear to be sufficiently high to have any significant effect on the formation of self-gravitating clumps. Self-gravitating clumps form rapidly in the models both with and without the flux-limiter.

These models suggest that the reason for the different outcomes of numerical models of disk instability by different groups cannot be attributed solely to the handling of radiative transfer, but rather appears to be caused by a range of numerical effects and assumptions. Given the observational imperative to have disk instability form at least some extrasolar planets, these models imply that disk instability remains as a viable giant planet formation mechanism.

Abstract: We investigate the tidal interaction between a low-mass planet and a self-gravitating protoplanetary disk, by means of two-dimensional hydrodynamic simulations. We first show that considering a planet freely migrating in a disk without self-gravity leads to a significant overestimate of the migration rate. The overestimate can reach a factor of two for a disk having three times the surface density of the minimum mass solar nebula. Unbiased drift rates may be obtained only by considering a planet and a disk orbiting within the same gravitational potential. In a second part, the disk self-gravity is taken into account. We confirm that the disk gravity enhances the differential Lindblad torque with respect to the situation where neither the planet nor the disk feels the disk gravity. This enhancement only depends on the Toomre parameter at the planet location. It is typically one order of magnitude smaller than the spurious one induced by assuming a planet migrating in a disk without self-gravity. We confirm that the torque enhancement due to the disk gravity can be entirely accounted for by a shift of Lindblad resonances, and can be reproduced by the use of an anisotropic pressure tensor. We do not find any significant impact of the disk gravity on the corotation torque.

Migration and Final Location of Hot Super Earths in the Presence of Gas Giants

Authors: Ji-Lin Zhou, Douglas N.C. Lin

(Submitted on 1 Feb 2008)

Abstract: Based on the conventional sequential-accretion paradigm, we have proposed that, during the migration of first-born gas giants outside the orbits of planetary embryos, super Earth planets will form inside the 2:1 resonance location by sweeping of mean motion resonances (Zhou et al. 2005). In this paper, we study the subsequent evolution of a super Earth (m_1) under the effects of tidal dissipation and perturbation from a first-born gas giant (m_2) in an outside orbit. Secular perturbation and mean motion resonances (especially 2:1 and 5:2 resonances) between m_1 and m_2 excite the eccentricity of m_1, which causes the migration of m_1 and results in a hot super Earth. The calculated final location of the hot super Earth is independent of the tidal energy dissipation factor Q’. The study of migration history of a Hot Super Earth is useful to reveal its Q’ value and to predict its final location in the presence of one or more hot gas giants. When this investigation is applied to the GJ876 system, it correctly reproduces the observed location of GJ876d around 0.02AU.

Abstract: We present the results of hydrodynamic simulations of the formation and subsequent orbital evolution of giant planets embedded in a circumbinary disc. We assume that a 20 earth masses core has migrated to the edge of the inner cavity formed by the binary where it remains trapped by corotation torques. This core is then allowed to accrete gas from the disc, and we study its orbital evolution as it grows in mass.

For each of the two accretion time scales we considered, we performed three simulations. In two of the three simulations, we stop the accretion onto the planet once its mass becomes characteristic of that of Saturn or Jupiter. In the remaining case, the planet can accrete disc material freely in such a way that its mass becomes higher than Jupiter’s. The simulations show different outcomes depending on the final mass m_p of the giant. For m_p=1 M_S (where M_S is Saturn’s mass), we find that the planet migrates inward through its interaction with the disc until its eccentricity becomes high enough to induce a torque reversal. The planet then migrates outward, and the system remains stable on long time scales. For m_p > 1 M_J (where M_J is Jupiter’s mass) we observed two different outcomes. In each case the planet enters the 4:1 resonance with the binary, and resonant interaction drives up the eccentricity of the planet until it undergoes a close encounter with the secondary star. The result can either be ejection from the system or scattering out into the disc followed by a prolonged period of outward migration.

These results suggest that circumbinary planets are more likely to be quite common in the Saturn-mass range. Jupiter-mass circumbinary planets are likely to be less common because of their less stable evolution.

Abstract: Two consecutive transits of planetary companion OGLE-TR-111b were observed in the I band. Combining these observations with data from the literature, we find that the timing of the transits cannot be explained by a constant period, and that the observed variations cannot be originated by the presence of a satellite. However, a perturbing planet with the mass of the Earth in an exterior orbit could explain the observations if the orbit of OGLE-TR-111b is eccentric. We also show that the eccentricity needed to explain the observations is not ruled out by the radial velocity data found in the literature.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last nine years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

On Comments

Centauri Dreams publishes selected comments on the articles under discussion here. The primary criterion is that comments contribute meaningfully to the debate. Among other criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations. A fuller statement of the policy can be viewed on the Administrative page.